Childhood asthma causes substantial morbidity and health care costs. Of the 18 million asthmatics in the United States, roughly 10 million are children under the age of 16. Asthma is the most common admitting diagnosis to pediatric hospitals in the United States, and asthma accounts for 2.8 million office visits per year in the United States. It is estimated that the cost of asthma care in children accounts for over $6 billion in health care costs per year. We propose to conduct a prospective longitudinal cohort study to examine the role of allergen exposure in the home on the development of two main outcomes: 1) asthma/wheeze during infancy and early childhood; 2) allergic sensitization, as measured by skin test reactivity (during early childhood). The primary allergen exposures of interest include house dust mite (der p I, der f I), fungi, cockroach (bla g I), and cat (fel d I) antigens. Additional exposures will be examined that may impact the association between allergens and the two main outcomes. These exposures include: heredity, assessed via family history of allergy and/or asthma, and via cord blood IgE; in utero and postnatal tobacco smoke exposure; acute lower respiratory illness; gender; race; socioeconomic status; and perinatal factors. If allergen exposure proves to be important in the prediction of sensitization and the development of asthma/wheeze in early life, it is a finding of great public health significance, as control of exposure measures could be instituted which could potentially minimize sensitization and exposure and, hence, the development of the disease.